Implement more granular cluster-merging

TODO: Documentation.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=71445

util/options.hh

diff --git a/util/options.hh b/util/options.hh
index 8b9b10e..6eb6c04 100644
--- a/util/options.hh
+++ b/util/options.hh
@@ -180,6 +180,7 @@
     num_features = 0;
     shapers = NULL;
     utf8_clusters = false;
+    cluster_level = HB_BUFFER_CLUSTER_LEVEL_DEFAULT;
     normalize_glyphs = false;
     num_iterations = 1;
 
@@ -202,6 +203,7 @@
 			 (bot ? HB_BUFFER_FLAG_BOT : 0) |
 			 (eot ? HB_BUFFER_FLAG_EOT : 0) |
 			 (preserve_default_ignorables ? HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES : 0)));
+    hb_buffer_set_cluster_level (buffer, cluster_level);
     hb_buffer_guess_segment_properties (buffer);
   }
 
@@ -265,6 +267,7 @@
   unsigned int num_features;
   char **shapers;
   hb_bool_t utf8_clusters;
+  hb_buffer_cluster_level_t cluster_level;
   hb_bool_t normalize_glyphs;
   unsigned int num_iterations;
 };